A variety of devices and applications are present which require cooling of devices that have very high heat fluxes, such as in the range of 100-1000 W/cm2. These devices include integrated electronic circuits in microprocessors, laser diodes, and power semiconductor devices for control electronics. There have been many solution strategies for cooling these devices.
One solution strategy for cooling a device emitting high heat fluxes include utilizing a heat pipe 10 having a vapor chamber 12, as shown in FIG. 1A. The heat pipe 10 includes a wick structure 14 which draws liquid to the heat source 99 by the use of capillary forces. In particular, as shown in FIG. 1A, the liquid evaporates in the wick 14 when heated and the resulting vapor escapes to the center of the heat pipe 10 where it is propelled to cooler regions for condensation. However, a problem with the geometry of the heat pipe 10 is that the flowrate of the liquid is limited by the capillary pressure available for drawing liquid back into the wick 14. One way to increase the flowrate of liquid through the heat pipe 10, is to make the wick structure 14 thicker. However, thickening the wick structure 14 increases the heat transfer resistance for conduction normal to the wick structure 14 itself, thereby rendering the wick 14 less effective. The temperature rise between the heat inlet and the heat exchange interface would increase if a thickened wick 14 is used, thereby making the heat pipe 10 less effective.
Another solution strategy for cooling the high heat fluxes in the devices is using a microchannel heat sink 20 coupled to a pump 22 and a heat rejector 26, as shown in FIG. 1B. This approach in FIG. 1B achieves a much higher liquid flowrate per unit volume than heat pipes 10 (FIG. 1A) due to the presence of the pump. This approach increases the heat removal capacity of the heat sink 20 without increasing the system volume. The heat transfer resistance remains low, because the resistance is governed by the small hydraulic diameter and large surface-to-volume ratio of the microchannels 24 in the heat sink 20, which remains the same. Microchannel heat sinks 20 with two-phase boiling convection achieve high rates of cooling with relatively low flowrates through evaporation of the fluid.
However, a major problem with cooling a device using these two-phase microchannel heat exchangers is the large pressure gradients that occur along the channels when the liquid begins to boil. It is known that the vapor phase of a substance is much less dense than that of the substance in liquid form. Therefore, for a given pumping power, the vapor phase of the substance will accelerate through a channel by up to a factor of a 1000 times. The acceleration and the resulting shear forces of the vapor substance through the channel dramatically increases the pressure drop along the channel. The large pressure drop in the channel thereby causes two-phase unsteady flow instabilities along the channel. These instabilities are assisted with bubbles forming in the flow and large drag forces being produced due to the small dimensions of the channels. The large pressure drop also greatly increases the amount of power required to pump the liquid through the microchannel heat sink 20. In effect, the microchannel requires more pumping power to cool a device 99, because the boiling of the liquid causes a very large increase in volume flow rate and a large pressure drop within the microchannel heat sink 20.
What is needed is a device which offers high flowrate capabilities, low thermal resistance and volume as well as has a phase separation capability in the heat exchanger which minimizes the pressure drop created by the phase change of the cooling liquid.